In drilling wells for the purpose of producing oil or gas from subterranean deposits, a fluid known as drilling mud is used to perform several functions necessary to successfully complete each well. This drilling fluid or mud performs many functions such as lubricating the drill string, cleaning the bore hole, and exerting sufficient pressure to the sides of the bore hole to prevent the entrance of liquids or gases into the bore hole from the formation being penetrated.
The drilling fluid must have a low fluid loss to prevent excessive loss of fluid into the formation by depositing an impervious filter cake on the sides of the bore hole. The thickness of the filter cake is usually directly proportional to the volume of fluid loss. Therefore, the lower the fluid loss the thinner the filter cake. Maintaining the diameter of the bore hole being drilled is critical to a successful operation. If the fluid loss is high, then the wall cake will be thick and therefore, the desired diameter of the well bore will be reduced.
Fluid loss additives most commonly used to control the fluid loss and also the wall cake thickness are bentonite clays, polymers, lignites, and surfactants. Gilsonite, a native asphalt occurring in vein deposits below the surface of the ground, greatly reduces fluid loss and wall cake thickness when properly incorporated with any water based drilling fluid.
Gilsonite is an asphaltic material that is found in Utah and Colorado. Gilsonite and other asphaltic-type materials penetrate the shale pore spaces as the drill bit penetrates the formation. It is assumed that a plastic-flow mechanism will allow the asphaltite to extend into the pores of the shale, thus, reducing fluid loss or mud invasion with a tendency to bond the shale and prevent sloughing. Asphaltite plates out on the bore hole to thereby reduce fluid loss.
However, asphaltite is by nature extremely hydrophobic and will not readily mix with water or water based drilling fluids. Thus, it is difficult to use asphaltite as an effective drilling fluid additive.
Attempts have been made to make asphaltite-based products more compatible with the drilling fluid. However, none of these attempts has been fully successful.
Moreover, in typical drilling mud systems, the asphaltic material is packaged in fifty pound bags and dumped into the mud hopper on the rig in amounts equaling from 1 to 50 pounds per barrel of mud. Since the asphaltic material is extremely hydrophobic, a surfactant is then added to the mud system in amounts of 0.5 to 10 per cent by volume to make the asphaltite disperse or become water wet. However, this process is extremely expensive because the surfactant might be used up on other solid materials in the mud system, such as, barite, bentonite, and drilled solids.
Furthermore, the above mentioned process is very expensive because it does not allow the asphaltite enough retention time with the aqueous phase of the drilling fluid to become dispersed or broken up into individual particles. Therefore, much of the material is lost over the rig shaker after the initial or first circulation through the well bore. Rig shakers can now be operated with shaker screens as fine as 250 mesh with 80-100 mesh being standard. With the above methods of adding asphaltite products dry to the drilling fluid, a conservative estimate of at least 10% and up to 90% of the asphaltite product is screened out and lost over the rig shaker after the first circulation.